The main factor was the size of the Communist Russia and then Soviet Union. Regardless of the death rate, natural or otherwise, people are born every year (youngest conscripts in 1941 were born after the Great War, Civil War and Polish-Bolshevik War), and with some exceptions, the birth rate is usually at least somewhat larger than the death rate, meaning that the natural population number (not factoring migration in) is growing over time. Today, this factor is relatively low, and sometimes even negative, but in the period of European history we're talking about (1917-1945) it was generally positive, with a notable exception of France. In other words, the population numbers of Russian Empire, Communist Russia and Soviet Union was naturally increasing.

This increase of the population was, of course, counteracted by the man-made disasters plaguing the last years of Russian Empire, and the early age of Soviet Union. The Great War, Civil War, failure of 'wartime communism', mass collectivization (especially in early 1930s) and political purges of late 1930s definitely took their toll, but although the number of their victims were substantial, the population base was more than able to recover the losses. Number of births in USSR was roughly in the ballpark of 49-52 per 1000, with an average fertility rate of 6.8 in mid-1920s, gradually decreasing to 4.2 in 1940. This means that the population was rising, on average, by about 2.5 mln every year, with natural death rate already being factored in. The Great Famine resulted in several millions deaths (the exact amount is unknown to this day, with the estimates ranging from 4 to 10 million), while the Great Purge resulted in about 1 mln deaths. Thus, these events, although significant, especially on local level (there were regions in what is now Ukraine and Kazakhstan, where population dropped by 25% or more between 1932 and 1933), did not have an extensive impact on the general population of USSR, amounting to roughly 3% and 0.7% of the total population respectively.

While consulting the raw population data we need to remember that the territory of the Russian Empire and Communist Russia/USSR fluctuated in the discussed period, chiefly because of the restitution of Poland in 1918 that claimed a part of former Russian territory as well as independence of Finland, Lithuania, Latvia and Estonia, what changed again with the recapture of the Baltic States and eastern territory of Poland in 1939-1940. Nevertheless, the population of Russian Empire in early 1914, shortly before the Great War was roughly 160 mln people and in the course of Great War, territorial cessions that resulted in formation of the aforementioned countries and the Civil War it fell to roughly 147 mln in 1926, but then, despite the disastrous collectivization in Ukrainian SSR and Stalin's purges in late 1930s, it rose to 170 million in 1939, prior to the breakout of the Second World War. In 1940, after the aforementioned annexations it achieved to 194 million (what was 2.5 more than 1941 population of Germany). It is also worth noting that the total casualties during Second World War are generally estimated to about 26 million, but the population of the USSR within the 1938 borders returned to pre-war levels around 1950, i.e less than 5 years after the war ended.

So, to sup it up, the population of the USSR was large enough to make up for the losses caused by wars and disasters between 1914 and 1941 that claimed relatively small percentage of the total inhabitants, especially in comparison with the number of deaths in Second World War.

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Andreyev, E.M. et al., Население Советского Союза, 1922—1991 [Population of the Soviet Union, 1922-1939], Nauka, Moscow 1993.